Back a United Front to Fight and Defeat Pests

An evaluation of last season’s challenges is a critical part of planning for success in the upcoming season. Among the most important considerations to review is the farm’s Pesticide Resistance Management Plan so effective pesticides remain working assets.


Farmers and crop consultants develop their Integrated Pest Management Plan (IPM) as a strategy combining a series of coordinated, cost-effective pest management methods to suppress pests below a defined aesthetic or economic threshold while having minimal impacts on non-target organisms and the environment. These IPM plans vary according to geographic region and crops grown but all are scientifically-based and regard pesticides as a critical component in the last line of defense against pests.

Today’s highly-valued crops coupled with heavy pest pressure from the subtropical South Florida environment requires an IPM program to include multiple control tactics to mitigate pest damage and maintain crop quality standards. The first line of defense against pests generally begins with principle of excluding pests from the cropping system. IPM plans also include everything from the principles of cultural practices and good sanitation to crop scouting, proper pest identification, and threat assessment, to the management decision, and management of pesticide resistance. Additional information may be found in the UF/IFAS EDIS Publication #IPM-200, Introduction to the Growers IPM Guide for Florida Tomato and Pepper Production by N.C. Leppla.

Exclusion tactics aim to prevent or at least delay disease development and insect establishment. Early onset of pest infestation can be reduced through the use of physical barriers like plastic, mulch, or dark heavy-fabric ground covers blocking sunlight and preventing seed establishment. Nets and fine-screens may also be utilized in the field and in structures like hoop-houses, shade-houses, and greenhouses to impede pest entry. Another exclusionary practice that may be employed is inspecting young seedlings rigorously and removing sickly plants before contamination spreads from greenhouse transplants to the field. Rogueing out such threats can mean the difference between replanting the crop at a total loss and containing the issue as a minor setback. Similarly, exclusion could also include purchasing weed-free and disease-free seed. This sort of tactic can sometimes be regarded as a helpful recommendation to mitigate pests but other times, they are required by state or local regulations.

This is just such the case for lettuce seed planted in the Everglades Agricultural Area of Palm Beach County due to a history of combating lettuce mosaic virus (LMV). This serious virus disease of lettuce has been declared a nuisance and a major economic threat to the production of lettuce in Florida. Lettuce seed and/or plants moved into, sold or planted in a commercial lettuce production area in Florida must be certified as having been tested for lettuce mosaic virus by a testing facility approved by the Division of Plant Industry. Additional information on state regulations regarding lettuce and LMV can be found in CHAPTER 5B-38 (LETTUCE MOSAIC) of the Florida Administrative Code and Florida Administrative Register. Further details about south Florida’s lettuce industry may be found in the UF/IFAS EDIS Publication # CIR1460, Florida Crop/Pest Management Profile: Lettuce by Mark A. Mossler and Esther Dunn.

Exclusionary tactics can only go so far in their efficacy; no crop system can remain pest-free for long. Even in an enclosed greenhouse, microscopic fungal spores float on air currents and may enter through vents. Workers may inadvertently bring plant pathogens into the structure on their hands or the bottom of their shoes and insect pests on their clothes. The same methods of introduction work just as well in a field grown crop. Complicating matters is the weed seed present in the soil and established weeds already harboring viruses, insect pests, and other plant pathogens. Insect pests like whitefly and thrips have been biding their time all summer long on weeds, ornamentals, and other alternate hosts. The same is true of plant pathogens too. Many common weeds, such as American black nightshade (Solanum americanum P. Mill) for example, are also hosts of plant viruses capable of infecting vegetable crops and causing reduced quality. Cucumber mosaic virus, Pepper mottle virus, Tobacco etch virus, and Tobacco mosaic virus are among the many viruses nightshade may harbor and under the right conditions be vectored into certain vegetable crops like tomato, pepper, and eggplant. Information regarding how to manage night shade may be found in the UF/IFAS EDIS Publication #HS1176, American Black Nightshade Biology and Control in Fruiting Vegetables, Cucurbits, and Small Fruits by Andrew W. MacRae.

Alternate hosts may be managed if detected but many viruses have unidentified or asymptomatic weed hosts. It is truly a race against time to get the crop up and established before pests and pathogens do. Anytime new seed or transplants are brought into a system, they serve as new sources of food for the local pest populations. Insect pests, plant pathogens, and wildlife respond to the opportunity of new sources of nourishment and move to take advantage of these grower inputs. Consequently, the second principle component of most IPM plans includes cultural control practices that can manipulate environmental conditions so crop needs are satisfied and pests are less attracted to the crop.

Some pests are attracted to stressed plants which are also often more prone to disease than their healthy counterparts. Growers often alter the soil pH by incorporating amendments prior to planting and utilize nutrient analyses to determine fertilizer application rates and timing so crop stress is minimized. Just as important, is avoiding crop growth which if too rapid, results in spindly plants with long internodes and excessively soft foliage which is attractive to a wide array of pests including aphids and others with piercing-sucking mouthparts.

Since good growing conditions are so critical, soil and bed uniformity become essential for irrigation and fertilizer placement, rate and timing so all plants in the field receive equal inputs and crop uniformity is maintained. Humidity which often plays a role in disease development can be reduced by adjusting crop density. Inside protected structures like greenhouses, vents may be opened to reduce humidity. These protected structures may also utilize cooling fans to reduce air temperature and crop stress. Another way growers may manipulate the farmscape environment is using reflective mulch which makes it difficult for insect pests to locate the crop. This is particularly useful at the beginning of the season as seedlings cast little shade and are lost in the glare of bright sunlight flying insects see. As the plants grow and shade more and more of the reflective plastic, the efficacy of this tactic lessens. In some cases, reflective mulch is impractical and other cultural practices need consideration. Such is the case in south Florida with sugarcane and vegetables where crop rotation is utilized to allow pest pressure to subside.

Sugarcane is the king crop in the Everglades Agricultural Area of South Florida. The planting of other crops, namely vegetables, largely revolves around which fields are coming out of sugarcane production. Generally, after a four-year cycle of harvesting, enough sugarcane pests have accumulated in the soil to make it worthwhile to take the field out of sugarcane production. For a few months, the field may fallow and rest, it may be flooded for pest and soil subsidence management, or be planted with a rotational vegetable crop. When practicing crop rotation, the importance of sanitation may come into play since plant debris from the previous crop can negatively impact the crop to be planted. This is often the case when rotating between crops of the same family. Rotating between tomato, pepper, and/or eggplant is one such example since all of which are in the Solanaceae and susceptible to like pathogens. Another such example is the rotation between the various Cole crops in the Brassicaceae like cabbage, cauliflower, broccoli, Brussels sprouts and kale. On the other hand, ubiquitous pests may impact a variety of unrelated crops like wireworms which feed on developing sugarcane roots and the seedlings of many different vegetable crops. Additional information about managing wireworms and other pests through crop rotation may be found in the UF/IFAS EDIS Publication #171, Florida Crop/Pest Profile: Sugarcane by Mark Mossler.

The sanitation component of an IPM plan involves acknowledging plant debris and soil may contain pests and plant pathogens which should be eliminated before they move into the new crop. Again, this applies to weeds which may be harboring pests from the previous crop or season. For that reason, it is advisable to examine weed populations before removing them. Appling herbicide to insect-infested weeds may eliminate the weed host but the surviving insect pests may soon move into nearby crops seek new food source. Scouting perimeter weeds may reveal the necessity for an insecticide and herbicide tank-mix to prevent such a situation from occurring.

As the crop develops, pest infestation is just a matter of time in south Florida’s subtropical environment. It is generally advisable to move equipment and personnel from young plantings to older ones to reduce the likelihood of introducing pests to newly planted crops which usually have fewer pest issues than older plantings. It is not uncommon for pests like mites and thrips to hitch a ride on clothing or nematodes and weed seed to be transported by soil-encrusted tires. In some cases, certain viruses are more easily spread by people and tools than by insect vectors.

One such hazard in tomato production and one particularly troublesome in greenhouses is the threat from Tobacco mosaic virus (TMV). As the name suggests, TMV also infects tobacco in addition to over 350 other plant species. Workers may unwittingly introduce the virus into the cropping system in the tobacco products they carry. Air-cured tobacco products often used in the making of cigars may carry TMV. Flue-cured tobacco, used in making cigarettes, is heated repeatedly during processing, thereby inactivating most if not all TMV particles. Likewise, smoke from burning tobacco does not spread the virus but when tobacco products are handled or kept in pockets, hands and clothing can become contaminated with TMV and serve as a source of introduction and spread. In fact, the most important way TMV is be spread from plant to plant is on workers’ hands, clothing or tools. This so-called ‘mechanical’ transmission occurs when plants are handled ever so slightly damaging tiny leaf hairs and outer plant cells which leak sap onto tools, hands and clothing. TMV is capable of surviving outside the plant in dried sap on tools and other surfaces. If a TMV plant is handled and then a door opened with that hand, TMV is now on the door handle. The next person to open the door can pick up the TMV and spread it to any plant that they touch. Additional information about weeds as alternate hosts for viruses may be found in the UF/IFAS EDIS Publication # ENY-863, Common Weed Hosts of Insect-Transmitted Viruses of Florida Vegetable Crops by Gaurav Goyal, Harsimran K. Gill, and Robert McSorley.

There are of course exceptions to every generalization and the best way to get an indication of what is going on in the field is to scout the crop and surrounding area thoroughly. Fields and crops should be examined with the goal of detecting pest threats early and often enough to apply management tactics so crop damage is minimized. The frequency of scouting may change over the life of the crop depending on pest severity and damage capability. This is just such the case of scouting sweet corn in south Florida. During the vegetative stage for example, scouting two days per week may be frequent enough to manage the primary pests consisting of an assortment of caterpillars and leaf pathogens. As silks develop however, scouting frequency generally increases to three days per week due to the continuing threat from the worms and the corn silk fly. Knowing the crop and the regional pest threats and pressure can help determine scouting frequency. Additional information about corn silk fly management may be found in the UF/IFAS EDIS Publication # EENY-224, Cornsilk Fly (suggested common name), Euxesta stigmatias Loew (Insecta: Diptera: Otitidae) by Gregg S. Nuessly and John L. Capinera.

In addition to scouting frequently, the scouting record must be accurate and should answer the following questions:
• Which pests are present?
• What is the size of the pest population?
• Which stage(s) of the pest is present?
• How are the pests distributed through the crop?
• What is the crop stage?

Local knowledge of crops, pests and environmental conditions can help determine what threats to look for and during which crop stages to look for them in but sometimes more information is required. Symptoms may be unique or nonspecific. A wilted plant for instance could be the result of any number of factors like drought, insects or disease. Weak and suspicious looking plants should be investigated and examined carefully. With South Florida regularly experiencing more than its fair share of newly introduced pests, it critical to properly identify what is observed before making a management decision.

There are a multitude of resources available to aid in identification. Some are more useful than others and it is important to remember that many disease symptoms are similar in appearance and the wrong identification may result in an ineffective management decision that ultimately wastes time and money and puts unwarranted pesticides into the environment. UF/IFAS Extension publications and personnel are available as resources to aid in pest identification. Distorted foliage, for instance, could just as likely be from herbicide drift as from a feeding pest. Only in the case of the later circumstance might a pesticide application be recommended. Similarly, some symptoms of nutrient deficiency may be confused with virus or other disease symptoms.

Correct insect identification is further complicated by the various stages in the insect’s lifecycle. It may be advantageous to know what the adult form looks like and even better, though difficult at times, to recognize the eggs and immature forms. For instance, beet and fall armyworms are primary pests of sweet corn in south Florida and elsewhere. Recognizing the adult or moth form of the pest while walking through the field can alert you to a potential infestation before any feeding damage has occurred. Scouting the plants closely may reveal the presence of egg masses, given they are identifiable, they may indicate when and what will be hatching-out.

Further complicating scouting and reporting is that many pests are too small to identify with the unaided-eye and many others require specific laboratory analysis to determine. One such example of the later situation is with the whitefly B and Q biotypes that are both present in Florida again. It came to light earlier this spring that Biotype Q has resurfaced in Florida after being last detected in 2005. The Q Biotype is of particular interest because it has a wide host range, is being found outside of greenhouses for the first time, and has a propensity for developing pesticide resistance. Florida farmers should be aware of the Q Biotype and know there is effective whitefly pesticides labeled for vegetables. Growers should report any difficult to manage whitefly populations to their local extension agent for biotype identification since the B & Q Biotypes look identical to each other. Submitted samples for identification should be placed in an alcohol filled vial and kept out of the heat. Additional information on whitefly management can be found in the UF/IFAS EDIS Publication #ENY735, Management of Whiteflies, Whitefly-Vectored Plant Virus, and Insecticide Resistance for Vegetable Production in Southern Florida by Philip A. Stansly, Hugh A. Smith, Dakshina R. Seal, Eugene McAvoy, Jane. E. Polston, Phyllis R. Gilreath, and David J. Schuster.

Once the crop and surrounding area have been scouted and pests correctly identified, it is time to communicate the observations to the crop manager to determine what the next management tactic should be. A multitude of information goes into such a decision. Among the many considerations a crop manager makes before deciding which tactic(s) to implement next are: crop maturity, pests, diseases and beneficials present, the stage they are in, size of their populations, how they are distributed throughout the crop, field conditions, the weather forecast, the crop market, how the new scouting report compares to the previous one and what actions were already implemented.

Researchers and farmers alike have developed thresholds to use as an aid in deciding whether or not a pesticide application is necessary. The economic threshold is the point at which the pest level results in an economic loss occurs. Whereas the action threshold is the point at which an action must be taken so reaching the economic threshold avoided. Due the high value of fresh vegetable crops such thresholds are generally low. Additional information on scouting and using thresholds may be found in UF/IFAS Publication #ENY685, Scouting for Insects, Use of Thresholds, and Conservation of Beneficial Insects on Tomatoes by David J. Schuster and Hugh A. Smith.

Scouting allows pest populations below the action threshold to be monitored for development over time. It also allows for consideration of biological management tactics and use of pesticides should pest populations or disease severity increase to actionable levels. Once an action is warranted, it is important to consider which pesticides were previously used due to the possibility of developing a pesticide resistant pest population.

Pesticide resistance is a shift in the genetics of a pest population that allows individuals within a previously susceptible population to survive. Resistance may develop in all agricultural pest groups –insects, mites, fungi, bacteria, nematodes and weeds. Repeating applications of like pesticides can lead to selection of individuals with resistance. Such resistant pest populations have inherited traits that reduce their susceptibility to individual pesticides or groups of pesticides. This is an evolutionary process, survival of the fittest. When one pesticide or group of pesticides is used repeatedly in the same place, against the same pest, the surviving pest population may be more resistant to the pesticide than the original population was since resistant genes are being selected for.

In order to maintain effective pesticide chemistries, it is recommended to use pesticides selectively. Use them only for the pests that have reached the action threshold. It is also advisable to utilize biorationals which are generally soft or even nontoxic to beneficials. Above all else, read and obey the pesticide label. Use the product at the recommended rate and follow the resistance management instructions. Whether the pesticide is an insecticide, fungicide or herbicide, each is grouped according to the active ingredient’s mode of action. These group numbers developed respectively by the Insecticide Resistance Action Committee (IRAC), Fungicide Resistance Action Committee (FRAC) and Herbicide Resistance Action Committee (HRAC), provide an essential tool for the development of insecticide resistance management strategies. UF/IFAS EDIS Publication #PI-83, IRAC’s Insecticide Mode of Action Classification by F. M. Fishel may be reviewed for additional information on the rationale behind the Insecticide Resistance Action Committee’s (IRAC) mode-of-action classification.

It is critical for each of us to do our part to implement such strategies in order to maintain effective chemistries for future seasons. Especially considering pests have historically developed resistance to many types of pesticides and the future of pesticide availability is uncertain.